rtc: pcf8523: don't return invalid date when battery is low

[Baruch Siach <baruch@xxxxxxxxxx>]

Hi stable kernel team,

Please consider upstream commit ecb4a353d3afd45 ("rtc: pcf8523: don't
return invalid date when battery is low") for stable backport. As the
commit log explains, this fixes bogus system date when RTC backup
battery is low.  With this fix date is set to the well known 01-01-1970
value instead.

This commit should be applicable to v4.9+.

Thanks,
baruch

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